WASHINGTON 
Inch by inch, voter by voter, Barack Obama and John McCain labored for more than a year to lock down supporters and woo defectors. It turns out, though, that the nation's voters were a lot more fickle than commonly expected, and far more prone to switch allegiances.

An Associated Press-Yahoo News poll that tracked the same group of about 2,000 adults throughout the long campaign reveals a lively churning beneath the surface as people shifted their loyalties – some more than once.

Count Republican Kelly Townsend of Abilene, Texas, among them.

Townsend, 50, saw McCain as a "good person," and supported him early on. But eventually she abandoned the Arizona senator in favor of Obama.

As for McCain, she concluded, "I just think he's hot-tempered and a little bit of a warmonger."

Over the long haul, 17 percent of those who eventually voted for Obama had expressed support for McCain at least once in a series of 10 AP-Yahoo News polls conducted since November 2007, before the party primaries began. And 11 percent of McCain's eventual supporters had backed Obama at least once.

Conventional political wisdom says it can be pretty much taken for granted that most voters lean sharply left or right and commit to one candidate early on, and the real campaign fight is over a small slice of undecided voters in the middle.

But like much of the conventional wisdom in this anything-but-typical election year, that may be wrong. Election polls that showed only gradual shifts in support for Obama and McCain were masking a much more volatile electorate. Few voters made unwavering, long-term commitments to either candidate.

"In fact, things are a lot less locked down than they might appear in public," said Bill McInturff, who was McCain's campaign pollster. "And it's why you don't stop campaigning."

Just 28 percent of those saying they voted for Democrat Obama, and 27 percent saying they backed Republican McCain on Election Day, said they would vote for that party's candidate in all 10 AP-Yahoo News polls.

And just half of those surveyed supported their ultimate choice unwaveringly in AP-Yahoo News polls since June, when both parties' nominees were known. Just six in 10 of both Obama's and McCain's voters backed their contender in those surveys without exception since September, when the general election campaign began in earnest.

That's a contrast with most public opinion surveys. They showed Obama with a modest lead for much of the summer, McCain taking a slight edge after the two party conventions, then Obama building a telling advantage into the fall as the economic situation spiraled downward.

Those abandoning one candidate were often canceled out by others gravitating to him, resulting in little net change in the candidates' overall support. Yet the frenetic, beneath-the-radar movement helps explain why the two political parties spent hundreds of millions of dollars this year. They needed to constantly woo new supporters while keeping those they thought they already had from defecting.

Stephen Ansolabehere, a Harvard University political scientist who has studied voting behavior, said such movement has been especially pronounced lately. He cited Republican defections because of unhappiness with President George W. Bush and the war in Iraq, uncertainty over which party could best address the economic meltdown and this year's influx of young and other first-time voters.